Certainly enjoyed your blog Matt, its got me wondering what future projects are ticking over in your head.
As has been said...harden them again, but I suggest you look up 'Normalising' ( Thermocycling as they say in the US) and 'Soaking'.
Whenever you heat steel or work it you increase the grain structure as Mac said and make it brittle. Tempering will not solve this problem.
99% of broken knives I see are because the maker has omitted this stage. Either it a bushcrafter, who has followed online instructions, that can all be traced back to one bloke who didn't normalise when making a knife from a file, or it a maker who has decided to make his stock removal knives look like they have been forged by hammering the heated steel.
Fire steels suffer from this even more than knives, because they have to be forged . They will certainly have enlarged grain and then are left in a fully harden state.
There is lots of stuff on the net that explains how self taught blacksmiths finally discover this step and no longer have the problem of clients dropping and shattering their steels.
Basically every time a traditional blacksmith heats and works a carbon steel, he will, before heat treating, heat the steel to cherry and let it cool slowly to all colour has gone. A bit like annealing, but it doesn't have to cool as far, Some guys do this a few times and have tricks or theories of their own. All of them though will return the steel to a fine grain that if as an experiment was snapped, couldn't be seen with a naked eye.Next time someone says their knife has snapped, ask them if they can see grain,
Soaking , is another step that seems to be missed out in our internet bushcraft world. Lots of us start with files and springs, liking the idea of recycling and cheap materials, but soon turn to O1 steel as files are unreliable. O1 comes flat in any width, ready annealed and can just need shaping , hardening, then tempering.
If though its soaked for 10 mins at cherry ( its quenching colour as this varies between steels) the edge will be noticable better. Cherry by the way is the moment steel cools just enough to regain its magnetism, not when it is non-magnetised.
I can't emphaise enough how important a step normalising is and would like to see it return to a normal part of smithing in bushcraft.
Here are a few more tips as we seem to be travelling down the same path
Temper in an oven. I do use a blow torch for small tools, but again steel improves dramatically by being soaked at this stage. Some guys even Temper a few times in a kitchen oven. Roughly for O1 its an hour at 220C and files you might do at 240 or250C . Look at the steel manufacturers info sheets for details.
Softening the spine can be done with a torch as you have done but you still need to temper the edge to at least a dark straw colour.
If you don't have a hardeness tester" beast "your blades. Abuse them on hard woods and knots before you but a handle on , because then it easy to reheat treat.
Two other ways to keep the spine soft, and get a hamon as well, are to either partially quench the blade or to put fire clay where you want to keep the steel soft.
Finally forget files and springs in favour of old circular saw blades. The older the better, folk gasp when they first try them. Even better, forget trying O1 and go straight for Silver Steel . I love Silversteel as a wood carver, its not used much over here as it comes in round bar and needs to be forged flat, most Finnish knives are made of it and here it is so cheap.
Anyway enough from me,I could rabbit a bout steel for hours, I look forward to seeing your next projects
John
